The invention relates to a metal-ceramic substrate and a method of making a metal-ceramic substrate. The substrate is made up of at least two ceramic layers and a layer separating the two ceramic layers made up of metal.
Metal-ceramic substrates for electric and electronic circuits or modules, i.e., and particularly printed circuit boards for such circuits or modules, are known in different embodiments.
Also known is the “DCB process” (Direct Copper Bond technology), hereinafter also referred to as DCB bonding, which is used to bond metal layers or sheets (e.g. copper sheets or foils) with each other and/or with ceramic or ceramic layers, namely using metal or copper sheets or foils, which are provided on their surfaces with a layer or coating (hot-melt layer) resulting from a chemical bond between the metal and a reactive gas, preferably oxygen. In this method, which is described, for example, in U.S. Pat. No. 3,744,120 and in DE-PS 23 19 854, this layer or coating (hot-melt layer) forms a eutectic with a melting temperature below the melting temperature of the metal (e.g. copper), so that the layers can be bonded to each other by placing the foil on the ceramic and heating all layers, by melting the metal or copper essentially only in the area of the hot-melt layer or oxide layer.
This DCB process includes the following steps:                oxidation of a copper foil so as to produce an even copper oxide layer;        placing the copper foil on the ceramic layer;        heating the composite to a process temperature between approx. 1025 and 1083° C., e.g. to approx. 1071° C.; and        cooling to room temperature.        
Also known is the so-called active soldering method (DE 22 13 115, EP-A-153 618) for bonding metal layers or metal foils, in particular copper layers or copper foils, with ceramic material. In this process, which is used especially for manufacturing a metal-ceramic substrate, a bond is produced at a temperature of ca. 800-1000° C. between a metal foil, for example a copper foil, and a ceramic substrate, for example aluminum-nitride ceramic, using a hard solder, which in addition to a main component such as copper, silver and/or gold also contains an active metal. This active metal is at least one element of the group Hf, Ti, Zr, Nb, Ce, creates a bond between the solder and the ceramic through a chemical reaction, while the bond between the solder and the metal is a metallic hard solder bond.
“Bonding” as used in the invention means generally connecting, e.g. DCB bonding, active soldering, hard soldering, adhesive bonding, etc.
The term “essentially” as used in the invention means deviations from the exact value by +/−10%, preferably by +/−5% and/or deviations in the form of changes that are insignificant for the function.
It is an object of the invention is to present a metal-ceramic substrate with improved mechanical, thermal and electrical properties.